An important function that must be provided in high quality optical networks is that of monitoring the status of individual channels of the network. Typically efficient optical transmission systems use a number of channels, each supplied by a laser of a particular wavelength, and such channels are combined for transmission over a common transmission waveguide, such as a broad-band optical fiber.
In such a network, it is necessary to measure the wavelength of the laser providing the light beam that serves to make a particular channel unique, for example to insure that the wavelength difference between adjacent wavelength channels is sufficient to permit reliable separation of the channels. Also it is generally necessary to measure the power level of the signal to insure that it remains high enough to provide a signal-to-noise ratio sufficient to provide reliable fidelity of transmission.
A common technique for doing the desired monitoring is to periodically sample the wavelength and the power level of the signal in a particular channel to make sure these are within the design parameters.
An important tool for combining and separating signals of various channels in wavelength-division multiplexing optical systems has been the wavelength router. The wavelength router may be viewed ideally as an arrangement for producing a wavelength dependent matching between an input signal applied to an input port of an input reference circle and an output signal derived from an output port at an output reference circle.
The basic geometry of a wavelength router includes a first free-space slab component, such as a star coupler, configured to receive radiation from various input ports spaced therearound, and a second similar free-space slab component configured for transmitting radiation applied to its various inports to various output ports spaced therearound. An optical grating comprising an array of waveguides, or arms, couples the two free-space slabs. Generally the length of adjacent arms of the grating differs by a constant amount and the arms are uniformly spaced apart along the boundaries of the free-space slabs. The number of arms included in the grating is dependent on the number of channels in the signals, and is generally at least several times the number of channels. Ordinarily there will be at least twenty arms in a router and typically more than thirty, and often more than one hundred.
Typically, a router will have an input port that is supplied by an optical fiber carrying a plurality of different wavelength channels. These channels will be separated and transferred to different output ports, allowing them to be transported to different destinations. Moreover, the router may be provided with a plurality of input ports, each connected to a separate fiber carrying a number of wavelength channels, for distribution of the channels to output ports.
As is known, various different transfer characteristics can be realized in a router between the input ports and the output ports depending on the wavelengths of the signals and the spacings and dimensions of the input and output waveguides involved.
Additionally wavelength routers of the kind described might be used for time-division multiplexing a plurality of signals, all of the same wavelength, that are supplied to different input ports, appropriately spaced in time, to be available at a common output port appropriately multiplexed.